Devices for measuring the flow rates of powdery and granular materials as they flow through a gravity flow system are well known in the art. The inventors herein are also the inventors of U.S. Pat. No. 4,069,709 which disclosed such a device. Another dry flow sensor device is also disclosed in U.S. Pat. No. 3,640,135. These patents disclose devices which generally include an inclined plate which is placed in the flow path of the material to be metered. A mechanical spring means of some sort is used to resist movement of the plate, and a transducer is mechanically coupled to the plate to produce an electronic signal proportional to plate deflection which may be amplified and displayed to provide a measurement of the sensed rate of flow of the material. As can be appreciated, each of these prior art systems require deflection of the plate to produce a measurement of the flow rate. Also, a mechanical spring action resists movement of the plate such that the resisting force is linearly related to the magnitude of plate deflection. Furthermore, these systems are "open loop" systems in the sense that there is no feedback which compares the measured flow rate with the position of the plate to correct for errors in measurement. Instead, the linear deflection is measured by a transducer and translated into a flow rate.
As the plate in these devices must deflect to obtain a reading, and mechanical springs are used to resist that deflection, the mechanical springs are subject to taking on a "set" or zero position which varies from their initial configuration. This "set" results in a shifting of the zero or null position for the flow rate sensor, and an error in the indicated flow rate. Furthermore, as Hook's law governs the resistive force of a spring, i.e., F=distance X spring constant, a linear relationship exists between the deflection of the plate and the resisting force. Also, the frictional losses in the system, including the spring, are greater for greater travels of the plate. Therefore, there is a Hobson's choice between a spring having a greater spring constant which provides less deflection and hence less accuracy in the measurement of the flow rate, and a spring having a smaller spring constant providing greater deflection and greater accuracy, but with greater chance of zero shift and frictional losses in the spring and system. These factors have greatly limited the available accuracy for flow rate sensors of this design in the prior art.
Still another problem with the prior art devices was the tendency for them to clog with material. As the amount of deflection of the plate is generally small, and its movement is dampened, material has a tendency to gather on and around the plate which throws off the accuracy of the sensor, and could also clog the chute. As the sensors were commonly mounted in chutes which might be a substantial distance above a floor, the dry flow sensor would fall into disuse merely from a neglect to adequately clean the device.
To solve these and other problems, the inventors herein have succeeded in developing a linear force transducer and suspension system therefor which is adaptable to the dry flow sensor application, among others. The transducer includes a unique leaf spring suspension to support an actuator rod extending to the plate with a servo mechanism connected to the actuator rod to resist deflection of the plate (in several embodiments eliminating deflection) and an indicator which displays the magnitude of the generated restoring force as an indication of the applied force, and hence the flow rate sensed by the plate. As a servo mechanism is used, there is closed loop feedback which significantly improves the accuracy of the flow rate sensor. As noted above, the prior art device merely displays the indicated flow rate. With the present invention, the indicated flow rate is a direct measure of the restoring force needed to balance the forces on the plate. If the restoring force is greater than the applied force, an imbalance occurs which lessens the restoring force through action of the servo. Thus, a feedback loop is provided which ensures that the restoring force exactly matches the applied force to much more accurately measure the applied force, and hence the flow rate.
Disclosed herein are three embodiments of a servo system. One may generally be referred to as the "speaker" design as it incorporates the structure from a conventional loudspeaker. The cone of the speaker is cut away from the surrounding frame, and the actuator rod is secured to the voice coil such that current through the voice coil produces the restoring force. The voice coil is supported by and is free to move with the actuator rod which is itself suspended by the suspension system. A second design, or "solenoid" design, incorporates a pair of solenoids which are connected in opposition to each other and to the actuator rod such that an imbalance between the coil currents produces a net force on the actuator rod. Still a third design is the "magnetic spring" design which can be used with lower flow rates producing lower forces on the plate. It includes an electromagnetic coil having a cylindrical opening through its center, with a soft iron core extending therethrough which is secured to the actuator rod. Thus, current through the coil produces an electromagnetic force along the axis of the iron core which resists the force applied to the plate.
In addition to these several embodiments of the servo, there is disclosed herein several embodiments of electronic circuits for measuring and amplifying the applied force, generating a restorative force, and energizing the servos. One such circuit is the "electronic spring" circuit which measures the deflection of the actuator rod to produce an input to an amplifier circuit, the amplifier circuit output being used to energize the servo. In this circuit, a minimal deflection of the plate, actuator rod, and transducer is required to achieve amplifier output. In a second or "true servo" circuit, deflection of the plate and actuator rod is sensed and a resistive force generated forcing the plate and actuator rod to their zero or null positions. Unlike the "electronic spring" circuit, this circuit requires only an initial, instantaneous deflection of the plate to achieve an amplifier output, and the plate is returned to its null position. Actually, the circuit is designed with very fast response times to maintain the actuator rod and plate essentially at their zero or null positions by integrating the applied force and applying it to the plate. In a third circuit, a programmed microprocessor provides great versatility in processing the applied force signal and generating the restorative force. This results in the plate being more closely maintained in its zero or null position during rapid changes of flow rate, and, hence, rapid changes of forces impinging on the plate. By being fully programmable, the microprocessor circuit permits the transducer to be adapted for a wide range of materials and operating conditions, and a selection of various parameters to optimize performance.
Each of these electronic restorative force generating circuits include features which enhance the operation of the linear force transducer in a dry flow sensor application. For example, a "dithering" voltage, or full wave rectified, un-filtered A-C is applied to the servo to set up a minute oscillation of the plate to overcome the mechanical state friction of the plate and its suspension so that any minimal applied force will deflect it. Still another part of the circuit occasionally applies an oscillating current of sufficient magnitude and frequency to the servo to heavily "shake" the plate and knock off any accumulated material. This effectively cleans it and prevents clogging of the chute in which the plate is mounted. A velocity sensing device may be placed in the chute to detect the velocity of the falling granular material, and its output used to adjust the indicated flow rate up or down for material having different particle sizes. Also, an accelerometer may be mounted on the chute, in line with the actuator rod's axis, to detect vibrations of the chute. These vibrations may then be subtracted from the measured applied force.
Still another feature of the invention is the unique suspension which substantially limits the movement of the actuator rod to a linear motion and which is virtually frictionless in that there are no sliding bearing members. Generally, the suspension includes a pair of spaced, multiple-leaf springs, each spring having three leaves of equal length. The center leaf is twice as wide as the outer leaves, and the center leaf of each spring is connected to the actuator rod with the outer leaves connected to the supporting structure. Thus, as the actuator rod moves, the leaves of the springs separate equally about a centerline drawn through the hinge point joining the tops of the leaves. Furthermore, as the leaves separate, the hinge point migrates towards the actuator rod which ensures that the actuator rod moves linearly along its axis, and not arcuately around the hinge. Each leaf spring may be constructed of a one piece spring element, or assembled from various components having greater or lesser degrees of resiliency. In addition to the actuator rod, this same suspension system may be used to mount the plate, or force sensing surface against which the flow of material impacts. This virtually frictionless suspension also serves to strictly limit the movement of the deflector plate to a linear motion which can be aligned with the actuator rod to ensure a correct transmission of the applied force.
With the dry flow sensor of the present invention, measurements may be taken which approach 0.1% accuracy in the indicated flow rate, where devices of the prior art were only capable of 5% accuracy. This increased accuracy is very important in the weighing and loading of dry materials both in processing applications, and in packaging or packing applications.